Filter circuit

ABSTRACT

A filter circuit, preferably a low-pass filter, whose cut-off frequency is easily controlled and which can be manufactured as an integrated circuit. First and second transistors are connected in differential amplifier configuration, and a current source is connected to the common connection of the emitter electrodes thereof. The base electrode of the first transistor is connected to an input terminal. A reactance element is connected between the collector electrode of the second transistor and a source of reference potential, such as ground. A third transistor has its emitter electrode connected to the base electrode of the second transistor and its base electrode connected to the collector electrode of the second transistor. An output terminal is connected to the emitter electrode of the third transistor.

BACKGROUND OF THE INVENTION

This invention relates to a filter circuit and, more particularly, to a filter circuit whose cut-off frequency is easily controlled and, moreover, which can be constructed as an integrated circuit.

It is conventional to fabricate filter circuits as integrated circuits. For cost effectiveness in manufacturing such integrated circuit filters, it is important that the number of external connections which are needed for proper operation thereof, that is, the number of connecting terminals which must be provided, be minimized. Heretofore, active filters have been manufactured as integrated circuits. However, in typical IC active filters, the resistance values of the various resistive elements often cannot be as high as desired. This results in a non-uniform cut-off frequency. That is, in a particular run or batch of IC active filters, the cut-off frequency of one may differ from that of the other.

Another difficulty found in IC active filters is that, since the temperature characteristic of the resistive elements often is less than satisfactory, the cut-off frequency of the filter is, to a significant extent, dependent upon temperature. Thus, the operating characteristics of the IC active filter may become unstable with temperature deviations.

Yet another disadvantage in typical IC active filters is that a low cut-off frequency for either a low-pass filter or a high-pass filter is not easily obtainable because the resistance and capacitance values of the resistive and capacitive elements therein are not as high as desired. That is, limitations on the resistive and capacitive values prevent the filter from having a relatively low cut-off frequency.

OBJECTS OF THE INVENTION

Therefore, it is an object of the present invention to provide an improved filter circuit which overcomes the aforenoted difficulties and problems attending prior art IC active filters.

Another object of this invention is to provide a filter circuit whose cut-off frequency can be controlled easily, and which can be manufactured as an integrated circuit.

A further object of this invention is to provide a variable filter circuit whose cut-off frequency is controlled as a function of a control signal.

An additional object of this invention is to provide a filter circuit which exhibits a controllable cut-off frequency such that manufactured filters of different batches all can be controlled to exhibit substantially identical operating characteristics.

Yet another object of this invention is to provide an improved filter circuit whose operation is relatively unaffected by temperature.

A still further object of this invention is to provide a low-pass filter circuit whose cut-off frequency may be made desirably low.

Various other objects, advantages and features of the present invention will become readily apparent from the ensuing detailed description, and the novel features will be particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

In accordance with this invention, the filter circuit is provided with first and second transistors connected in differential amplifier configuration having their emitter electrodes coupled in common to a current source. An input terminal is connected to the base electrode of the first transistor. A reactance element, such as a capacitor, is connected between the collector electrode of the second transistor and a source of reference potential, such as ground. A third transistor has its emitter and base electrodes connected to the base and collector electrodes respectively, of the second transistor. An output terminal is connected to the emitter electrode of the third transistor. Various embodiments of the filter circuit are disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description, given by way of example, will best be understood in conjunction with the accompanying drawings in which:

FIGS. 1-9 are schematic representations of various embodiments of the filter circuit in accordance with the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings, wherein like reference numerals are used throughout, and in particular to FIG. 1, there is illustrated a basic embodiment of a filter circuit in accordance with the teachings of the present invention. The circuit shown herein is comprised of transistors Q₁, Q₂ and Q₃.

Transistors Q₁ and Q₂ are connected in differential amplifier configuration with their emitter electrodes connected in common. The common connection of these emitter electrodes is coupled to ground by a controllable current source A₁. The base electrode of transistor Q₁ is connected to an input terminal T₁ and the collector electrode thereof is connected to a terminal T₃ supplied with an operating potential +V_(cc).

A reactance element, shown herein as a capacitor C, is connected between the collector electrode of transistor Q₂ and a source of reference potential, such as ground, to serve as an AC load. It will be appreciated that, if desired, other reactive elements, such as an inductance, may be used in place of capacitor C. A current source A₂ also is connected to the collector for electrode of transistor Q₂.

Transistor Q₂ has its base and emitter electrodes connected to the collector and base electrodes, respectively, of transistor Q₂. The collector electrode of transistor Q₃ is connected to terminal T₃. Another current source A₃ connects the emitter electrode of transistor Q₃ to ground; and this emitter electrode also is connected to an output terminal T₂.

In operation, let it be assumed that the input voltage applied to terminal T₁ is represented as V_(in), and let it be further assumed that the output voltage provided at terminal T₂ that is, at the emitter electrode of transistor Q₃, is represented as V_(out). With current source A₃ connected to the emitter electrode of transistor Q₃, it is appreciated that this transistor exhibits an emitter-follower configuration. Hence, if the base-emitter voltage drop across this transistor is neglected, the voltage at the base electrode thereof is substantially equal to the voltage at the emitter electrode thereof. Thus, the voltage provided at the base electrode of transistor Q₃ is substantially equal to V_(out).

Let it be assumed that the angular frequency of the voltage provided at the terminal T₁ is represented as ω. As a result of this signal, a signal current i_(s) flows through the common-connected emitter electrodes of transistors Q₁ and Q₂. The resistance in this path traversed by the signal current is equal to the emitter resistances of these transistors. If each emitter resistance is represented as r_(e), and if these resistances are equal, then the signal current i_(s) is equal to the voltage difference between the base voltages of transistors Q₁ and Q₂ divided by the emitter resistance in this circuit, or ##EQU1## This signal current i_(s) also flows through capacitor C to produce the output volage V_(out). As mentioned above, the base voltage of transistor Q₃ is substantially equal to V_(out). Accordingly, the voltage across capacitor C may be expressed as:

    V.sub.out =i.sub.s /jωC                              (2)

The transfer function H (ω) of the filter shown in FIG. 1 is equal to the output voltage divided by the input voltage. From equations (1) and (2), this transfer function may be expressed as: ##EQU2## The transfer function expressed in equation (3) indicates that the filter circuit of FIG. 1 functions as a low-pass filter whose cut-off frequency ω_(c) may be represented as:

    ω.sub.c =1/2Cr.sub.e                                 (4)

Now, if the current which flows through current source A₁ is represented as 2I, then, since the emitter currents of transistor Q₁ and Q₂ are equal, the emitter current of each transistor is equal to I. The emitter resistance r_(e) is related to the emitter current I in each of transistors Q₁ and Q₂ in accordance with the following expression: ##EQU3## wherein k is the Boltzmann constant, T is absolute temperature and q is the charge of an electron.

If equation (5) is substituted into equations (3) and (4), then the transfer function H(ω) and the filter cut-off frequency ω_(c) may be rewritten as: ##EQU4##

It is, therefore, appreciated that the cut-off frequency of the low-pass filter shown in FIG. 1 can be changed, or controlled, as a function of the magnitude of the current flowing through current source A₁. Various embodiments of current source circuits are known to those of ordinary skill in the art, and an example thereof is described in greater detail below. Since the current through a current source may be controlled by a suitable control signal, it is further recognized that the operating characteristics, that is, the transfer function and cut-off frequency of the filter circuit illustrated in FIG. 1 may be controlled in response to such a control signal. Consequently, since the cut-off frequency is established as a function of the current I, as is apparent from equation (7), and since this current can be controlled easily, the cut-off frequency ω_(c) from one filter circuit to another may be suitably controlled so as to be uniform. Furthermore, even though equation (7) represents that the cut-off frequency is dependent upon temperature T, it is appeciated that this temperature dependency can be cancelled by suitably controlling current I. For example, if the temperature increases, the current may be increased, in response to the aforementioned control signal, so as to balance, or cancel, any effect upon the cut-off frequency attributed to this change in temperature.

Another advantage of the circuit shown in FIG. 1 is that, even though the capacitance of capacitor C may be limited by reason of state-of-the-art integrated circuit fabrication techniques, the cut-off frequency ω_(c) may be made as low as desired merely by reducing the current I. Thus, it is seen that the embodiment shown in FIG. 1 overcomes the aforenoted disadvantages of prior art IC active filters and that the present invention is capable of providing low-pass filters of uniform cut-off frequency, of good temperature immunity and of low cut-off frequencies, as desired. Also, since the cut-off frequency is determined by the current of current source A₁, the cut-off frequency can be varied rapidly over a relatively wide range merely by controlling the current source. That is, as the current I changes, the cut-off frequency changes in the corresponding manner.

Yet another advantage of the embodiment shown in FIG. 1 is that the output impedance thereof is determined by the output impedance of transistor Q₃. It is appreciated that this output impedance is relatively low. Also, the DC potential at input terminal T₁ is equal to that at output terminal T₂. Consequently, the illustrated filter circuit can be connected to other circuits without undesired loading effects, and may be incorporated easily in a multi-stage device.

The signal current i_(s) is, of course, a function of the input voltage V_(in). As this signal current varies, the emitter resistance r_(e) of each of transistors Q₁ and Q₂ also may vary. However, by reason of the differential amplifier configuration of these transistors, it should be recognized that such variations in the emitter resistances thereof are in opposite directions. That is, if the emitter resistance of one transistor increases by reason of signal current i_(s), the emitter resistance of the other transistor decreases. Consequently, such a change in the emitter resistances because of a change in the signal current has a cancelling effect. As a result thereof, the apparent change in emitter resistance r_(e) caused by signal current i_(s) is negligible. This means that the illustrative filter circuit exhibits a good distortion factor and a wide dynamic range.

Another embodiment of the present invention is illustrated in FIG. 2. This embodiment differs from that shown in FIG. 1 in that current source A₂, which is coupled to the collector electrode of transistor Q₂, is constituted by a transistor Q₆ which is connected to a transistor Q₅ in a current mirror circuit configuration. Also, in the embodiment of FIG. 2, a particular example of current source A₁ is illustrated as comprising transistor Q₄ whose base electrode is supplied with a control voltage E via a voltage divider circuit formed of resistors R₁ and R₂. Control voltage E is applied to a terminal T₄ which, in turn, is connected to the voltage divider circuit.

When a current mirror circuit is used as current source A₂, as shown in FIG. 2, signal current i_(s) through capacitor C is doubled. That is, because of this current mirror circuit, signal current i_(s) exhibits twice the value in FIG. 2 than in FIG. 1 for the same input voltage V_(in). Consequently, the cut-off frequency ω_(c) for the embodiment of FIG. 2 is twice the cut-off frequency for the embodiment of FIG. 1, and may be expressed as: ##EQU5##

Of course, if control voltage E is varied, the collector current of transistor Q₄ is varied in response thereto. Hence, a change in the control voltage results in a change in current I so as to vary or adjust the cut-off frequency ω_(c).

From equation (8), it is seen that the cut-off frequency is dependent upon temperature T. For example, if the temperature increases, it would appear that the cut-off frequency ω_(c) decreases. However, in the embodiment of FIG. 2, since current source A₁ is constituted by transistor Q₄, it is recognized that the increase in temperature T results in an increase in the collector current of transistor Q₄. This means that, as temperature T changes, current I in equation (8) changes in a similar manner. Consequently, any dependency of the cut-off frequency on temperature is cancelled. That is, a reduction in the cut-off frequency caused by an increase in temperature is cancelled because current I also increases by a corresponding amount. Thus, in practice, the cut-off frequency ω_(c) is not temperature dependent to any significant extent and, therefore, the temperature characteristic of the illustrated filter circuit is improved over the prior art.

Another embodiment of the present invention is illustrated in FIG. 3. This embodiment differs from that described above with respect to FIG. 2 in that current source A₁ is not specified as being of any specific construction, although it may, of course, be constructed as shown in FIG. 2; and in that a series of n diodes is connected in the emitter circuit of transistor Q₁ and a series of n diodes is connected in the emitter circuit of transistor Q₂. As shown specifically, diodes D₁₁, D₁₂, . . . D_(1n) are connected in series between the emitter electrode of transistor Q₁ and current source A₁. Similarly, diodes D₂₁, D₂₂, . . . D_(2n) are connected in series between the emitter electrode of transistor Q₂ and the current source. Each diode exhibits a resistance r_(e). That is, the resistance of each diode is equal to the emitter resistance of transistor Q₁ (and also transistor Q₂). This equal resistance may be attained easily in accordance with conventional integrated circuit manufacturing techniques. Hence, the effective resistance between the base electrode of transistor Q₁ and current source A₁ is equal to (n+1) r_(e). Similarly, the effective resistance between current source A₁ and the base electrode of transistor Q₂ is equal to (n+1) r_(e). Thus, in the embodiment shown in FIG. 3, the resistance in the path traversed by signal current i_(s) is equal to 2(n+1) r_(e). With this value of resistance, the derivation of the mathematical expression for cut-off frequency ω_(c) results in the following: ##EQU6##

A comparison of equations (8) and (9) indicates that the cut-off frequency ω_(c) for the embodiment of the filter circuit shown in FIG. 3 will, for equal currents I and for equal capacitance C, be lower than the cut-off frequency for the embodiment of the filter circuit shown in FIG. 2. That is, the cut-off frequency for the embodiment shown in FIG. 3 is 1/(n+1) the cut-off frequency for the embodiment of FIG. 2. It is appreciated that if the cut-off frequency is to be the same for both embodiments, then the current I in equation (9) must be (n+1) times as great as the current I in equation (8). If the magnitude of the current I relative to the magnitude of signal current i_(s) is large, as in the FIG. 3 embodiment, the dynamic range of the filter circuit is increased. Thus, for equal cut-off frequencies, the dynamic range of the embodiment shown in FIG. 3 is wider than the dynamic range of the embodiment shown in FIG. 2.

Referring now to the embodiment shown in FIG. 4, it is recognized that this embodiment is substantially similar to that described above with respect to FIG. 3, except that the diode resistances r_(e) of the FIG. 3 embodiment are replaced by ohmic resistances R₃ and R₄ in the FIG. 4 embodiment. Hence, the FIG. 4 embodiment attains substantially the same results and exhibits substantially the same effects as the FIG. 3 embodiment. That is, for the same value of current I, and for the same value of capacitance C, the cut-off frequency for the embodiment of FIG. 4 is substantially reduced relative to the cut-off frequency for the embodiment of FIG. 2 dependent upon the values of resistances R₃ and R₄. Also, if the cut-off frequency ω_(c) for the embodiment of FIG. 4 is to be equal to the cut-off frequency for the embodiment of FIG. 2, then the current I in the FIG. 4 embodiment is relatively large as compared to its signal current i_(s). Hence, the dynamic range of the FIG. 4 embodiment is wider than the dynamic range of the FIG. 2 embodiment.

In all of the aforedescribed embodiments, the output voltage V_(out) provided at the emitter electrode of transistor Q₃ is fed back substantially in its entirety (that is, approximately 100 percent of the output voltage is fed back) to the base electrode of transistor Q₂. In the embodiment of FIG. 5, current source A₃ is formed by series-connected resistors R₅ and R₆ which, as is known, serve as a current source if the resistors exhibit a sufficiently high resistance value. In addition to functioning as a current source, resistors R₅ and R₆ serve as a voltage divider. Thus, a voltage-divided portion of output voltage V_(out) provided at the emitter electrode of transistor Q₃, which transistor is connected in emitter-follower configuration, is applied to the base electrode of transistor Q₂. The voltage dividing ratio K is equal to R₆ /(R₅ +R₆). Accordingly, the voltage which is fed back to the base electrode of transistor Q₂ is equal to KV_(out).

Since only a portion of the output voltage is fed back to the base electrode of transistor Q₂, the transfer function H(ω) and the cut-off frequency ω_(c) of the illustrated filter circuit both are functions of this ratio K. The transfer function and the cut-off frequency may, therefore, be expressed as follows: ##EQU7## From equation (11), it is recognized that, in accordance with the embodiment shown in FIG. 5, the cut-off frequency ω_(c) is reduced by an amount determined by the voltage-dividing ratio K relative to the embodiment discussed above with respect to FIG. 2. If, in the FIG. 5 embodiment, the cut-off frequency is to be equal to that of the FIG. 2 embodiment, then, from equation (11), it is appreciated that current I must be increased. Hence, the dynamic range of the FIG. 5 embodiment will be wider than that of the FIG. 2 embodiment.

Yet another embodiment of the present invention is illustrated in FIG. 6, wherein two stages of the low-pass filter shown in, for example, FIG. 2 are connected in cascade. One stage is comprised of transistors Q₁ -Q₆, interconnected in the manner described above with respect to FIG. 2. However, the input of this stage, that is, the base electrode of transistor Q₁, is connected to the output of a preceding stage which is comprised of transistors Q₁₁ -Q₁₆. Respective ones of the latter transistors correspond to transistors Q₁ -Q₆ and are interconnected in a similar manner. Thus, transistors Q₁₁ and Q₁₂ are connected in differential amplifier configuration with their common-connected emitter electrodes coupled to ground by current source transistor Q₁₄. The base electrode of transistor Q₁₁ is connected to input terminal T₁ to receive a signal to be filtered. Furthermore, a current mirror circuit formed of transistors Q₁₅ and Q₁₆ is connected to the collector electrodes of transistors Q₁₁ and Q₁₂, similar to the current mirror transistors Q₅ and Q₆ connected to transistors Q₁ and Q₂.

Capacitor C₂ in the preceding filter stage is connected between the collector electrode of transistor Q₁₂ and output terminal T₂ of the following stage. Thus, the "reference" potential to which capacitor C₂ is coupled is the output DC potential. Finally, transistor Q₁₃ is connected in a manner similar to transistor Q₃, in that the emitter electrode of transistor Q₁₃ is coupled to ground by a current source A₁₃ and also to the base electrode of transistor Q₁₂, and the base electrode of transistor Q₁₃ is connected to the collector electrode of transistor Q₁₂. The output of the first filter stage is derived from the emitter electrode of transistor Q₁₃ and, as mentioned above, is coupled to the input of the second stage, i.e., to the base electrode of transistor Q₁.

Transistors Q₄ and Q₁₄ serve as respective current sources, and also are connected with a transistor Q₇ in current mirror configuration. That is, the base electrodes of transistors Q₄ and Q₁₄ are connected in common with the base electrode of transistor Q₇, the emitter electrodes of transistors Q₄ and Q₁₄ are connected in common with the emitter electrode of transistor Q₇, and the collector electrode of the latter transistor is connected to its base electrode and also to terminal T₄ to receive a control voltage E applied thereat.

In the embodiment of FIG. 6, let it be assumed that the collector current of transistor Q₁₄ is equal to 2I₁, and the collector current of transistor Q₄ is equal to 2I₂. The cut-off frequency ω_(c) of these cascaded filters can be derived analogous to the foregoing derivations such that the cut-off frequency is: ##EQU8## The quality factor Q of this circuit may be expressed as: ##EQU9##

FIG. 7 illustrates another embodiment of the present invention which is similar to that just described with respect to FIG. 6, except that, rather than supplying 100% of the output voltage to the base electrode of transistor Q₂, a voltage-divided ratio K of this output voltage is supplied. It is seen that the second filter stage in FIG. 7 is analogous to the embodiment of FIG. 5. Thus, resistors R₅ and R₆ serve as a voltage-divider circuit having the ratio K (wherein K=R₆ /R₅ +R₆). Also, capacitor C₂ of the first stage is coupled to the junction defined by resistors R₅ and R₆ to receive K times the output DC potential.

The operation of the filter circuit shown in FIG. 7 is similar to that of FIG. 6, except that the cut-off frequency ω_(c) of the FIG. 7 embodiment is reduced by an amount that is a function of K. In the interest of brevity, and since one of ordinary skill in the art would recognize the transfer function and cut-off frequency relationships for this embodiment, further description of FIG. 7 is not provided.

FIG. 8 represents a filter circuit which is constructed of two cascaded low-pass filter circuits H₁ and H₂ which may be of the construction described hereinabove with respect to the embodiments of FIGS. 1-7. In the filter circuit of FIG. 8, the output terminal of filter H₁ is connected to the input terminal of filter H₂, and also to an output terminal T₆. The output terminal of filter H₂ is connected to an amplifier H₃ and also to an output terminal T₇. Amplifier H₃ feeds back a filtered signal to a subtracting circuit H₄, the latter also being connected to input terminal T₁ to receive an input signal to be filtered. The output of subtracting circuit H₄, which is equal to the difference between the input signal and the fed back amplified signal, is supplied to filter H₁ and also to an output terminal T₅.

Let it be assumed that the cut-off frequency of low-pass filter H₁ is higher than the cut-off frequency of low-pass filter H₂. If subtracting circuit H₄ is omitted, for the moment, then the higher frequencies of the input signal supplied to input terminal T₁ are filtered out by low-pass filter H₁. Thus, a lower frequency signal is passed by filter H₁ to low-pass filter H₂. Since the cut-off frequency of filter H₂ is assumed to be lower than that of filter H₁, only the lower frequencies of the filtered signal supplied to filter H₂ (that is, the frequencies below the cut-off frequency of filter H₂) are passed therethrough. Thus, amplifier H₃ is supplied with a lower frequency signal, the original higher frequencies of which having been filtered out.

Now, when subtracting circuit H₄ is considered, it is appreciated that the lower frequency signal which is provided at the output of amplifier H₃ is subtracted from the original input signal supplied to input terminal T₁. Thus, subtracting circuit H₄ effectively subtracts, or cancels, the lower frequency signals from the original input signal. Consequently, output terminal T₅ is provided with a high frequency signal which, of course, is the difference signal between the input signal and the lower frequency signal that is subtracted from the input signal.

Low-pass filter H₁ removes the upper frequencies from the signal provided at the output of subtracting circuit H₄. Thus, output terminal T₆ is provided with a higher frequency signal (from subtracting circuit H₄) whose upper frequencies are removed (by filter H₁), thereby resulting in a band-pass filtered signal.

Thus, the filter circuit illustrated in FIG. 8 functions both as a high-pass filter to supply a high-pass filtered signal at output terminal T₅ ; and also as a band-pass filter to supply a band-pass filtered signal at output terminal T₆.

Referring now to FIG. 9, there is illustrated a modification of the embodiment described above with respect to FIG. 1. In this modified embodiment, the base electrode of transistor Q₃ is connected to an input terminal T₈ via capacitor C for receiving an input signal V₂. This differs from the previously described embodiment wherein the base electrode of transistor Q₃ is coupled via capacitor C to ground. If input terminal T₁ is supplied with the input signal V₁, then the signal path traversed by signal current i_(s) is formed of input terminal T₁, the emitter resistances r_(e) of transistors Q₁ and Q₂ and capacitor C to input terminal T₈. The signal circuit i_(s) through capacitor C is equal to the signal current i_(s) through the emitter resistances of transistors Q₁ and Q₂ and may be expressed as: ##EQU10## The terms in equation (14) can be rearranged so as to solve for the output voltage V_(out) as follows: ##EQU11## wherein R=2kT/q.

The first term in equation (16) represents that the filter circuit shown in FIG. 9 exhibits low-pass characteristics in response to the input voltage V₁ supplied to input terminal T₁. The second term in equation (16) represents that the filter circuit exhibits high-pass characteristics in response to the input voltage V₂ supplied to input terminal T₈. The cut-off frequency for the low-pass characteristic is equal to the cut-off frequency for the high-pass characteristic. These cut-off frequencies are controlled as a function of the current 2I through current source A₁.

If it is assumed that V₁ =-V₂, then the transfer function H(ω) for the filter circuit shown in FIG. 9 may be expressed as: ##EQU12## The transfer function of equation (17) is seen to have a variable phase that is a function of the current I. Thus, the embodiment of FIG. 9 functions as a controllable phase shift circuit when V₁ =-V₂, the amount of phase shift being controlled by the current 2I through current source A₁.

It is appreciated that each of the filter circuits shown in the foregoing embodiments may be constructed as an integrated circuit. As is typical, an IC must be provided with external connecting terminals, or pads, by which it can be electrically connected to other circuitry. These connecting terminals have been shown, in the illustrated embodiments, as terminals T₁, T₂ . . . . The cost of construction of an IC is determined, to a large part, by the number of such external connecting terminals that must be provided. In the foregoing embodiments, the filter circuit is shown with a minimum number of connecting terminals. Hence, the present invention is readily adapted for low cost IC manufacturing.

While the present invention has been particularly shown and described with reference to various embodiments, it should be readily appreciated to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention. For example, although the reactance device used herein has been shown as a capacitor, it may, alternatively, be replaced by an inductor. If an inductive reactance device is used, the filter circuit exhibits high-pass characteristics.

Also, it is appreciated that, in the various embodiments described above, current source A₂ may be replaced by current mirror transistors Q₅ and Q₆, and vice versa. Furthermore, current source A₃ may be any conventional current source, such as a current source transistor, a relatively high resistance, or the like. Preferably, current source A₁ is formed as a current source transistor whose collector-emitter current is controlled by a suitable control signal so as to correspondingly control the cut-off frequency of the filter circuit.

It is intended that the foregoing, as well as various other changes and modifications, be included within the scope of the appended claims. 

What is claimed is:
 1. A filter circuit, comprising an input terminal for receiving an input signal; first and second transistor means, each having base, emitter and collector electrodes, said first and second transistor means being connected in differential amplifier configuration and having their emitter electrodes connected to a common connection; variable current source means connected to said common connection; said input terminal being connected to the base electrode of said first transistor means; a source of reference potential; reactance means coupled between the collector electrode of said second transistor means and said source of reference potential; third transistor means having base, emitter and collector electrodes, the base electrode of said third transistor means being connected to the collector electrode of said second transistor means and the emitter electrode of said third transistor means being connected to the base electrode of said second transistor means; an additional current source connected to the emitter electrode of said third transistor means; resistance means connected between the emitter electrode of each of said first and second transistor means and said common connection; and an output terminal connected to the emitter electrode of said third transistor means, whereby the cut-off frequency of said filter circuit is a function of the current produced by said variable current source.
 2. The circuit of claim 1 wherein said reactance means comprises a capacitor.
 3. The circuit of claim 1, further comprising another current source connected to the collector electrode of said second transistor means.
 4. The circuit of claim 3 wherein said other current source comprises one of a pair of transistors connected as a current mirror circuit, the other of said pair of transistors being connected in series with the collector electrode of said first transistor means.
 5. The circuit of claim 1 further comprising a current mirror circuit including a pair of transistors whose collector-emitter circuits are connected to said first and second transistor means, respectively.
 6. The circuit of claim 1 wherein said variable current source comprises an additional transistor; and means for supplying a current-control signal to the base electrode of said additional transistor to control the current flowing therein.
 7. The circuit of claim 1 wherein said resistance means comprises ohmic resistors.
 8. The circuit of claim 1 wherein said resistance means comprises n diodes connected to the emitter electrode of each said first and second transistor means, wherein n is an integer. 